Computing systems generally use one or more display monitors to provide a visual input/output capability. Such display monitors are similar in many respects to conventional television receiver displays. Thus, many technologies, including the present invention, may be applied effectively to both. In both systems, a cathode ray tube (CRT) includes an evacuated envelope usually made of high-strength glass. The envelope includes a generally flat or slightly curved faceplate or viewing screen together with a funnel shaped bell and extending neck. The interior side of the faceplate supports a phosphor screen. In monochrome displays, a single electron gun is supported within the CRT neck and is directed toward the phosphor screen. The electron gun produces a beam of electrons which are directed toward the faceplate striking the phosphor screen and causing visible light to be emitted therefrom. In color display systems, a plurality of electron guns are used together with a phosphor screen which supports plural areas of phosphors having differing color light emitting characteristics. A shadow mask or similar structure is interposed between the electron guns and the phosphor screen to cause each of the electron guns to stimulate an associated type of colored light emitting phosphor.
Whether the display system is monochrome or color, the electrons emanating from the electron gun or guns form a CRT beam which is scanned in both the horizontal and vertical directions across the faceplate to form a raster. In most instances, the horizontal scan system is operative at a higher frequency than the vertical scan system. Thus, the horizontal scan moves the electron beam rapidly from side to side across the faceplate while the vertical scan system causes the successive horizontal scans to be moved progressively from top to bottom to complete a display frame and form the raster.
In the majority of the presently used display systems, electron beam scanning is accomplished by electromagnetic deflection of the CRT beam. A deflection yoke is supported upon the CRT envelope between the electron guns and the faceplate. The deflection yoke supports a plurality of deflection coils which are coupled to the horizontal and vertical scan systems. Horizontal and vertical scan signals provided by the respective scan systems are coupled to the windings of the deflection yoke to produce corresponding electromagnetic fields which bend the electron beam and thereby direct it to the desired portion of the CRT faceplate. Both the horizontal and vertical scan signals include longer duration sloped scan portions followed by shorter duration high amplitude retrace portions. The latter are utilized at the completion of each respective scan interval to return the electron beam to its starting position. In addition, the retrace portion of the horizontal scan signal is used to develop the high voltage necessary to accelerate the electron beam toward the CRT faceplate.
The character of the image displayed in a CRT display system results from variation or modulation of the intensity of the scanned CRT electron beam. This intensity modulation must be properly timed or synchronized to the horizontal and vertical rate scanning of the raster. Thus, as the electron beam is scanned across the faceplate to form a raster, the desired portions of the faceplate are illuminated by synchronized modulation of the electron beam to provide the desired image.
One of the problems associated with CRT displays arises from the relationship between CRT beam current intensity, high voltage power, and deflection sensitivity. As CRT beam current intensity changes, the loading imposed upon the high voltage power system is also changed which in turn causes an inverse change in the high voltage potential. As high voltage potential is changed, the degree of electron beam bending which results from the electromagnetic fields of the deflection yoke (deflection sensitivity) is also changed. For example, an increase in CRT beam current imposes a greater load upon the high voltage system causing a reduction of high voltage potential. The reduction of high voltage potential produces a corresponding increase in deflection sensitivity (more electron beam bending) which in turn causes the raster to be enlarged or "bloom". Because the CRT beam current intensity modulation is synchronized to the horizontal and vertical scan, the displayed image is undesirably enlarged as the raster blooms.
In the event the changes in beam current exist for relatively long time intervals (e.g. several vertical scan periods), the entire raster becomes expanded and the displayed image is correspondingly enlarged. While this general enlargement of the display may be annoying, an even more deleterious effect results from abrupt relatively short term changes in beam current. Such shorter duration beam current changes cause localized or partial blooming and display enlargement. The result is that portions of the displayed image are expanded or shifted with respect to the remaining image elements.
To meet the difficulties associated with raster blooming in response to beam current changes, practitioners in the art have usually attempted to "stiffen" or regulate the high voltage generating system and render it less sensitive to beam current changes. The detailed structure of such systems varies substantially. However, all generally include some means for sensing CRT beam current and regulating the high voltage power in a compensating manner. The objective is to increase high voltage power as beam current increases, thereby maintaining a relatively constant high voltage. To the extent the high voltage potential is maintained constant, the above-described changes in deflection sensitivity in response to beam current and the resulting raster blooming are avoided.
An alternate approach to CRT beam current compensation involves compensatory scan amplitude changes to maintain image size. One such system uses a series power resistor interposed between the horizontal and vertical scan systems and their supplies of operating voltage. The objective of this approach, known as partial decoupling, is to tolerate high voltage changes as well as raster blooming and make compensating changes in the magnitudes of horizontal and vertical scan during raster blooming.
While the foregoing improvements provide some benefit in responding to the problem of raster blooming, they tend to be complex, expensive and/or inefficient. High voltage regulation requires a substantial reserve or excess of high voltage power and expensive regulation components. Partial decoupling of the deflection systems requires one or more power resistors and wastes substantial energy. In addition, the partial decoupling approach is further limited by the generally slow response of scan amplitude to CRT beam currents. As a result, changes of deflection amplitude do not properly track or correspond to changes of CRT beam current.
There remains, therefore, a need in the art for an inexpensive, efficient display size control system which effectively compensates for changes in CRT beam current intensity.
Accordingly, it is a general object of the present invention to provide an improved display system. It is a more particular object of the present invention to provide an improved display system which substantially maintains image size despite changes in image intensity. It is a still more particular object of the present invention to provide an improved display system which is efficient and relatively inexpensive.